= Viscous evolution of an accretion disk
\b[<Viscous evolution of an accretion disk>] follows the redistribution of <surface density> as internal <stresses> exchange <angular momentum> between rings. For an axisymmetric <Keplerian disk> without <mass loss>, combining <mass> and angular-momentum conservation gives $\partial_t\Sigma=(3/r)\partial_r[r^{1/2}\partial_r(\nu\Sigma r^{1/2})]$. Here $\nu$ is an effective <kinematic viscosity>. The equation describes both inward accretion and outward spreading; the sign of local <mass> flow follows from the radial gradient, rather than <viscosity> alone.<Dynamics of Astrophysical Discs by Gordon I. Ogilvie>[Lecture4, section4.2: diffusion equation for mass evolution]{ref}
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